Micromechanical devices, which are customarily integrated with analyzing circuits, are used, for example, in automobile manufacturing, machine controls and regulators, as well as in many areas of consumer electronics. For all areas it is essential that the components used and the respective analyzing circuits be cost-effective, reliable and highly functional.
Although the present invention is applicable in principle to any micromechanical device, the present invention elucidated with reference to a micromechanical swiveling mirror.
Micromechanical swiveling mirrors are used, for example, as switching elements in optical telecommunication technology, scanning elements for deflecting a laser beams for bar code recognition or area monitoring, or marking tools.
Using low-stress electroplating, micromechanical swiveling mirrors can be manufactured without warping, in particular, with respect to the mirror surfaces. Such metallic micromechanical components are usually manufactured using LIGA technology, 3D deep lithography or add-on methods.
In particular, the latter add-on, or additive, methods allow the size of the micromechanical arrangements and thus their price to be reduced, opening up new application possibilities. Inexpensive, reliable and durable micromechanical devices result. Additive methods also allow freely movable metallic structures to be created on any desired substrate, such as a silicon substrate, a glass substrate or a ceramic substrate.
Additive methods also allow large, unperforated surfaces to be exposed, so that solid mirror surfaces with dimensions of up to a few millimeters can be manufactured. This method can be mastered inexpensively and well as single-layer electroplating. Multiple electroplating can also be performed in order to manufacture the anchoring regions and the mirror surfaces or suspensions separately, for example. Large swiveling angles can be obtained using sacrificial layers of suitable thicknesses.
Electroplated metal structures normally contain an anchoring region provided on a substrate, in which a part of the corresponding device located over the substrate, for example, a structure that is freely movable over the substrate, is anchored in order to bond it mechanically and/or electrically to the substrate. The substrate should be understood as the base in the general sense.
FIGS. 3a-3g show cross-sectional views of the process steps of a manufacturing method according to the present invention for a micromechanical device.
In FIGS. 3a-3g, 10 denotes a substrate with an operating circuit after final processing, which has a passivation layer 15 with an open terminal pad 20 embedded therein. A sacrificial layer in the form of a first photoresist layer is denoted with 25; an adhesive layer in the form of a sputtered electroplating start layer (plating base) is denoted with 30; a second photoresist layer is denoted with 40; a silicon dioxide layer is denoted with 50; a third photoresist layer is denoted with 60 and an electroplated layer in the form of a nickel plating is denoted with 35.
The point of departure for manufacturing the micromechanical device according to the first embodiment of the present invention is the finished operating circuit with passivation layer 15 and open terminal pad 20.
As FIG. 3a shows, in a first step a first photoresist layer is applied as a sacrificial layer 25 and structured so that terminal pad 20 is exposed. This terminal pad 20 is used as the plating base for the anchoring region of the micromechanical device to be manufactured. First photoresist layer 25 can advantageously be used both for opening terminal pad 20 and as a sacrificial layer if terminal pad 20 must be initially opened in passivation layer 15.
As FIG. 3b shows, in a next step adhesive layer 30 is sputtered on in the form of an electroplating start layer (plating base). In this embodiment, this is a conductive layer made of chromium-copper. Chromium is responsible for the adhesion of first photoresist layer 25 under it; copper is used as the starting layer for the subsequent step of electrodeposition.
As FIG. 3c shows, an approximately 15.mu. thick second photoresist layer 40 is applied on adhesive layer 30 by centrifugation and set at temperatures typically around 200.degree. C.
Subsequently an approximately 600 nm thick silicon dioxide layer 50 is deposited on second photoresist layer 40 using plasma CVD (chemical vapor phase deposition). Silicon dioxide layer 50 is subsequently used as a hard mask for structuring second photoresist layer 40 under it and is structured for this purpose by a photolithographic process using a third photoresist layer 60 and by subsequent plasma etching, as shown in FIG. 3d.
After overetching silicon dioxide layer 50, trench etching of second photoresist layer 40 is performed using an anisotropic plasma etching process. The resulting structure is shown by FIG. 3e.
In the polymer negative form thus obtained, formed by second photoresist layer 40, a several micrometers thick nickel layer is electrodeposited. This results in the comb structure illustrated in FIGS. 3f and 3g. It should be noted that the individual areas of second electrodeposited layer 35 are connected in areas that are not visible in this cross sectional representation.
Subsequently silicon dioxide layer 50 is removed by wet chemical etching and the polymer negative form of structured second photoresist layer 40 is removed by dry chemical etching.
A selective wet chemical etching of adhesive layer 30 and etching of the sacrificial layer in the form of first photoresist layer 25 in plasma follows, resulting in the structure shown in FIG. 3g. Removal of sacrificial layer 25 in the form of the first photoresist layer is an isotropic etching process, with the photoresist under nickel combs 35 being completely removed.
The result is a micromechanical device with freely movable structures that can be operated as a capacitor, as FIG. 3g shows.
The drawback in the customary manufacturing processes is the fact that the anchoring regions must have a lateral design thickness of typically 30 .mu.m.times.30 .mu.m, since considerable lateral underetching occurs as adhesive layer 30 (plating base) is removed by selective wet chemical etching. This considerably limits the design options.